The invention relates to a method for the evaporation of monomers which are liquid at room temperature and have a low vapor pressure, especially those from the group TEOS (tetraethyl orthosilicate), TMS (tetramethylsilane) and HMDS (hexamethyldisiloxane), in the production of thin layers containing silicon and oxygen by chemical vapor deposition (CVD) in a vacuum on substrates, through the use of a flow adjuster having an evaporator connected to its output.
Plasma processes and vapor deposition processes are used at pressures below atmospheric pressure (vacuum) for producing coatings containing silicon and oxygen. The starting monomers are liquids, such as TEOS, TMS and HMDS, which are characterized by a relatively low vapor pressure at room temperature. On account of the extremely high purity required of these compounds, a so-called delayed boil often occurs when they are evaporated, i.e., an abrupt, explosive evaporation of relatively great amounts of liquid, and hence an irregular mass flow. This property is especially pronounced in the case of TEOS.
Through the article by B. L. Chin and E. P. van de Ven entitled, "Plasma TEOS Process for Interlayer Dielectric Applications," published in "Solid State Technology," Apr. 1988, pages 119 to 122, it is known to pump TEOS in liquid form into the vacuum reactor where it is evaporated and mixed with auxiliary gases. In spite of a computer-controlled liquid pump with a repeatable throughput it is not possible to produce a sufficiently constant mass flow on account of the intermittent evaporation of the liquid in the vacuum.
In considering these things it must be borne in mind that any irregularity in the mass flow is reflected in irregular coating properties and, if it is a reactive process that is involved, there will also be differences in the stoichometry of the coating composition.
Through the article by S. P. Mukherjee and P. E. Evans, "The deposition of thin films by the decomposition of tetraethoxysilane in a radio-frequency glow discharge," published in "Thin Solid Films," 1972, pp. 105-118, it is known to evaporate TEOS with a temperature-controlled evaporator which permits a defined increase of vapor pressure. In this case, too, continuous evaporation is virtually unachievable. If other monomers are to be evaporated, the apparatus must be carefully adapted to the vapor pressure curves of these monomers. Furthermore, the method in question is limited in its application, since it is not possible to achieve an arbitrarily high vapor pressure, and the easily condensable vapors can be transported and metered only in an unsatisfactory manner.
U. Mackens and U. Merkt describe a very similar method with comparable disadvantages in their article, "PLASMA-ENHANCED CHEMICALLY VAPOUR-DEPOSITED SILICON DIOXIDE FOR METAL/OXIDE SEMICONDUCTOR STRUCTURE ON InSb," published in "Thin Solid Films," 1982, pages 53-61.
Through the article by R. E. Howard, "Selecting Semiconductor Dopants for Precise Process Control, Product Quality and Yield and Safety," published in "Microelectronic Manufacturing and Testing," Dec. 1985, pages 20-24, it is furthermore known to pass a noble gas (argon, helium) through a liquid monomer contained in a quartz vessel, in which the noble gas entrains the monomers. Disadvantageous in this method is the lack of information on the amounts of the monomer actually reaching the reaction chambers. Furthermore, additional gases must always be put in, and in many applications the result is an unacceptable limitation in regard to the choice of the working parameters of pressure and gas flow.
Lastly, a method of the kind described above is disclosed in EP-OS 0 239 664, in which a needle valve is used as the flow adjuster and is connected to an evaporator not further described. The result of such an arrangement is that the monomer evaporates more or less uncontrolledly behind the needle valve which forms a constriction. The only purpose the evaporator then serves is to evaporate any entrained liquid droplets and thus to assure complete evaporation. Consequently a constant mass flow cannot be achieved even with this known method.
The purpose of the invention is therefore to devise a method of the kind described above, in which a constant, precisely controllable vapor flow or rate of flow can be sustained over a long period of time.